Assemblies of electrochemical units connected in series (often called stacks) are known. The electrochemical units thus assembled may be formed for example by accumulator elements, or by fuel cells. A fuel cell is an electrochemical device for converting chemical energy directly into electrical energy. For example, one type of fuel cell includes an anode and a cathode between which a proton exchange membrane is arranged, often called a polymer electrolyte membrane. This type of membrane only allows protons to pass between the anode and the cathode of the fuel cell. At the anode, diatomic hydrogen undergoes a reaction to produce H+ ions which will pass through the polymer electrolyte membrane. The electrons generated by this reaction join the cathode by a circuit external to the fuel cell, thus generating an electric current. Because a single fuel cell generally only produces a low voltage (around 1 volt), fuel cells are often series-connected to form fuel cell stacks able to generate a higher voltage comprising the sum of the voltages of each cell.
When used within the automobile industry, these fuel cell stacks are usually associated with a battery to form a hybrid system. This system connects the fuel cell stack and the battery in parallel so that the fuel cell stack and the battery simultaneously or separately power the car, via a common section called the bus. This hybridization also allows the fuel cell stack to recharge the battery. A hybrid system is called “active” when it uses a DC/DC converter connected at the fuel cell stack output as seen in FIG. 1. This DC/DC converter is used to adapt the voltage levels of the fuel cell stack and the battery and to regulate the power delivered by the fuel cell stack.
Regulating the power requires the implementation of a control strategy to distribute the power between the fuel cell stack and the battery according to the power requirement of the electric engine of the car and system constraints. System constraints which the control strategy has to take into account are the maximum voltages and currents of the fuel cell stack and the battery, the temperature ranges which must not be exceeded, the battery state of charge, i.e. for example, the battery must not be charged when it is already 100% charged, etc. . . .
One of the control strategies for this hybrid system consists in regulating the battery state of charge around a nominal value without ever reaching the maximum or minimum charge of said battery. Thus, the battery never needs to be charged externally, since it is recharged by the fuel cell stack and possibly by recuperating kinetic energy from the vehicle when the latter is in a braking phase. This means that the fuel cell stack supplies the mean power consumed by the electric engine of the vehicle, whereas the battery is used as an energy buffer means of charging or discharging energy. This strategy is implemented by regulating the bus voltage at a constant value using the DC/DC converter.
One drawback of this known strategy is that nothing is implemented to prevent the fuel cell stack from operating at open circuit voltage (“OCV”). “Open circuit voltage” means the area of operation in which the voltage per cell is higher than 0.85-0.9V/cell. This voltage is known to considerably reduce the lifetime of the fuel cell stack. It is therefore undesirable for the fuel cell stack to operate in this mode.
The open circuit voltage operating mode may occur when the fuel cell stack is only controlled with a constant pressure current. This control method is derived from the idea consisting in reducing the operating pressure of the fuel cell stack to low power to avoid the OCV range. However, it must be considered that the dynamics of pressure variation are much slower than the dynamics of current variation (on the order of a second for pressure and a millisecond for current). It must also be considered that a decrease in the fuel cell stack pressure can only occur if current is consumed, and the current value directly influences the pressure reduction speed. Thus, if the fuel cell stack power varies instantaneously (or quickly) from several kilowatts to 0 kW, it will not be possible to avoid the OCV range, since there will no longer be any current to reduce pressure and the fuel cell stack will be damaged.